NO310627B1 - Process for catalyzing by stream-free precipitation of metals on the surface of a non-conductive material - Google Patents

Description

The present invention relates to a method for catalysis by electroless precipitation of metals on the surface of a non-conductive material, e.g. plastic, glass, etc. The method includes capture of a catalyst metal that is strongly involved in the adhesion of a metallic deposit, or electroless precipitation, to a substrate by such an electroless deposition process. The invention relates in particular to a method for capturing and fixing a catalyst metal as a catalytic core on the surface of a substrate through selective and strong chemisorption of the catalyst metal from a solution containing the catalyst metal by the action of chitosan or a chitosan derivative contained in a pretreatment agent.

Non-conductive plasters, ceramic materials, paper, glass, fibres, etc., can be precipitated by electroless plating (hereinafter "precipitation"). In order to initiate oxidation of a reducing agent in a precipitation solution, however, the surface of such a non-conductive substance as mentioned above must be subjected to a catalysis treatment. Although a known classical catalyzing treatment method is a sensitizing-activating method using a stannous chloride bath and a palladium chloride bath, a catalyst accelerator method using a stannous chloride-palladium chloride bath and a sulfuric acid (or hydrochloric acid) bath is now generally used as a catalyzing treatment method. Furthermore, another catalysis treatment method has recently been adopted, whereby a substrate is immersed in a solution of a palladium complex which has a strong adsorbability and is then washed with water, followed by precipitation of palladium metal with a reducing agent such as dimethylaminoborane.

The pretreatment step useful for the catalyzing step is an etching step which is necessary to ensure a wettability (hydrophilicity) in the surface of a substrate to promote the physical adsorption thereon of a catalyst metal. In such an etching step, a chromic acid etching solution for plaster and the like is now used in most cases. In the chemical etching step, the substrate surface is given a microscopic roughening to facilitate the physical capture therein of a catalyst metal in the catalysing step while ensuring an anchoring effect by the adhesion of the resulting metal deposit, or plating, on the substrate. In this context, the chemical etching step is a very important step (see figs 1 and 2).

According to the sensitization-activation method, which is a two-step process, a substrate is first immersed in a solution of stannous chloride in the sensitization step of the process to adsorb Sn^4" on the surface of the substrate, and is then treated with a solution of palladium chloride in the activation step of the process to precipitate Pd- nuclei in accordance with a redox reaction represented by the following equation:

Chemicals to be used for sensitisation, i.e. sensitisers, have been studied from far back in time, including those described in the patent literature from around 1936 (US patent 2,063,034). The type of sensitizer is not often varied depending on the type of substrate and the type of electroless deposition. In the sensitizing step, a number of different hydrochloric acid-acidified solutions of stannous chloride are used, which are used alone as the main ingredient. Suggested sensitizing agents other than stannous chloride include platinum chloride and titanium chloride, which can also be used in the form of a hydrochloric acid acidified solution. On the other hand, a solution of palladium chloride (0.2-1 g/l, hydrochloric acid: 5 ml/l) is most widely used as an activation solution. Salts of noble metals such as Pt, Au and Ag other than Pd are also effective for an electroless copper precipitation solution.

The catalyst for use in the catalyst-accelerator method is a mixed solution of stannous chloride and palladium chloride with hydrochloric acid, and this is commercially available as a concentrated solution which is usually diluted with a large amount of a solution of hydrochloric acid to prepare for use. The catalyst-accelerator method is carried out at a treatment temperature in the range of 30-40°C for an immersion time of 1 to 3 minutes. 5-10 vol-% sulfuric acid or hydrochloric acid is usually used as an accelerator, and can alternatively be a solution of sodium hydroxide or ammonia. Rantell et al [cf. A. Rantell, A. Holtzman; Plating, 61, 326 (1974)] has reported that the mixed solution of stannous chloride and palladium chloride with hydrochloric acid is not colloidal, but a solution of a complex salt having the composition SvPdjCli^ and is solubilized in the presence of an excess of stannous chloride. Rantell et al further drew the following conclusion with regard to the course of a reaction in the accelerator step.

In the catalyst step, the Sn<2+->Pd<2+> complex salt is first adsorbed on the surface of a substrate and the adsorbed complex salt is then hydrolyzed when the substrate is washed with water. During the hydrolysis, tin is precipitated in the form of a Sn(OH)Cl precipitate, which coexists with tetravalent tin and the palladium salt. In the subsequent accelerator step, the precipitated stannous salt is dissolved and then reacted with the palladium salt which has already been freed from a state of a complex salt, so that palladium metal is obtained in accordance with the following redox reaction:

As a result, palladium metal and small amounts of divalent and tetravalent tin salts remain on the surface of the substrate.

The reaction mechanisms involved in the sensitizing-activation method and the catalyst-accelerator method, which are the conventional catalysis methods for electroless precipitation, therefore take place as discussed above. It will be seen that many reactions are involved until catalytic cores of a metal such as palladium are precipitated. The catalyst metal is consequently lost little by little in the form of various intermediate reaction products which are formed in the respective reactions every time a substrate is washed with water and the like for each such reaction. The final residue of catalyst metal is strongly influenced by many factors such as the concentrations, pH values and temperatures of solutions used in respective steps, the immersion periods for such solutions as well as the conditions for degreasing and roughening of the substrate surface. Thus, when the final uptake of catalyst metal becomes insufficient, the adhesion of the resulting metal deposit to the substrate is always imperfect so that errors in precipitation are caused.

The mentioned phenomena are due to pure "physical adsorption" of catalysis reaction intermediates and metal catalyst that are trapped in depressions and micropores in the surface part of the substrate that is microscopically roughened by chemical etching.

The purpose of the present invention is to provide a completely new method which can enable the achievement of a stronger adsorption bond of a catalyst to the surface of a substrate in order to achieve a great improvement in the adhesion of a metal deposit, or precipitation, on the substrate by the use of neither mentioned sensitization -activation method or catalyst-accelerator method.

According to the present invention, a method is thus provided for catalysis by electroless precipitation of metals on the surface of a non-conductive material, e.g. plastic, glass, etc., and this method is characterized by the formation of a coating film including chitosan or a chitosan derivative on said surface, and then treating the coating film with a solution of a salt of a catalyst metal to effect chemisorption thereof on said coating film.

Advantageous and preferred features of this method appear from the accompanying claims 2-4.

According to this method, the strong chemisorption of the catalyst metal on the coating film, which comprises chitosan or the chitosan derivative, enables an easy performance of electroless precipitation of metal on the surface of the non-conductive material.

Chitosan (P-1,4-poly-D-glucosamine) which can be used in the present invention is obtained by deacetylation of chitin (P-1,4-poly-N-acetylglucosamine) which is extracted as a natural polymer from shell and the like from crabs and the like. Chitosan is a cationic biopolymer that has amino groups, and is a new material that has useful properties such as moisture retention, antifungal properties and absorbency for heavy metals. Chitosan has a biological adaptability similar to chitin and its application in the pharmaceutical and biochemical area, such as artificial skin, is therefore thoroughly studied. The present invention has been arrived at by directing attention to the metal absorption capacity of chitosan, in particular a specific absorption capacity that chitosan has for precious metals such as palladium, platinum and rhodium. In addition to chitosan, chitosan derivatives such as carboxymethyl chitosan and glycol chitosans can also be used. The degree of deacetylation for chitosan or the chitosan derivative to be used in the present invention is preferably at least 80%, preferably at least 90%. When chitosan having a degree of deacetylation lower than 80% is used, it is possible that its adsorption capacity for a catalyst metal such as palladium, the hydrophilicity of the coating film, etc., are adversely affected.

Examples of the non-conductive material to be used as a substrate in the present invention are plaster, ceramic materials, paper, glass and fibres, which cannot be deposited directly by electrodeposition.

When forming an electroless precipitation of metal on the surface of the non-conductive material according to the present invention, the surface of the non-conductive material is coated with a treatment liquid containing at least chitosan or the chitosan derivative (hereinafter for the sake of simplicity referred to as "chitosan" ) to form a hydrophilic coating film on the non-conductive material's surface prior to the steps of catalysis and electroless plating. Chitosan which is in the thus formed hydrophilic coating film will chemically adsorb, capture and fix a catalyst metal such as palladium. In the electroless precipitation step, it is consequently possible to achieve such a condition that a sufficient amount of an active catalyst is created on the substrate surface on which the electroless precipitation which has a good adhesion to the substrate can be formed in a uniform and efficient manner (fig 3).

The chitosan concentration in the treatment liquid containing chitosan is preferably in the range 0.01-1%, preferably in the range 0.05-0.2%. When it is lowered below 0.01%, the concentration effect of the chitosan addition will be so reduced that effective capture of the catalyst is not achieved. On the other hand, when it exceeds 1%, the effect of the chitosan addition will become a saturation so that this reduces the coating effect of the treatment liquid.

In addition to chitosan, the chitosan-containing treatment liquid may contain a dilute acid such as acetic acid, formic acid or hydrochloric acid. The diluted acid can be used to dissolve chitosan in the treatment liquid. The concentration of the diluted acid is calculated on the basis of equivalence to the free amino groups in the chitosan compound to be used. Furthermore, the treatment liquid can in some cases be mixed with a resin that gives excellent adhesion to the substrate, although this depends on the type of substrate. Any resin can be used as long as it provides good compatibility or miscibility with chitosan. Examples of the resin that can be added to the treatment liquid include water-soluble resins such as polyvinyl alcohol and hydroxyethyl cellulose; water-solubilized resins of an alkyd, polyester, acrylic, epoxy or similar resin; and emulsions of a vinyl acetate, acrylic resin or similar resin. Chitosan itself can be cross-linked with polyethylene glycol diglycidyl ether or the like to make the coating film so stable that the catalysis reaction can be enhanced. Furthermore, in certain cases, a number of different inorganic pigments can be added to the treatment liquid to achieve a firmer adhesion to the resulting electroless deposit. In this case, in particular, the surface of the coating film formed by the application of the treatment liquid is microscopically uneven so that an anchoring effect is achieved during the electroless deposition formation to thereby further contribute to an improvement in the adhesion of the film to the electroless deposition, thus representing an alternative to the chemical etching step in the conventional processes. Useful examples of inorganic pigments are aluminum silicate, titanium oxide and barium sulphate. The amount of inorganic pigment may be in the range of 10-80%, preferably 50-70%, based on the solids content. When the amount exceeds 85%, the adhesion of the chitosan-containing treatment material to the substrate itself is reduced. Almost all remaining material is water and an organic solvent such as methanol, ethanol, isopropanol and/or ethyl acetate. Such a solvent is effective in improving the compatibility of various resins in case such materials are added, providing some erosion of the substrate and rapid drying of the treatment liquid after application thereof. In addition, a hydrophilic surface regulating agent can be added to the treatment liquid as needed to give the coating film formed from said liquid an appropriate degree of leveling and hydrophilicity. Examples of surface regulating agents are perfluoroalkylethylene oxides. The surface regulating agent can be added to an amount in the range of 0.05-1%, preferably 0.1-0.5%, based on the solid chitosan content.

Said treatment liquid containing chitosan can be applied to the substrate surface by a conventional application method such as spray coating, roller coating, brushing, or dip coating to form a coating film that can serve as a hydrophilic carrier for fixing the catalyst metal.

After formation of the catalyst metal-fixing carrier on the surface of the substrate, the step of fixing the catalyst metal through the catalyzing reaction is carried out followed by the step of electroless precipitation. An electroless deposition which has good adhesion to the substrate can thus be effectively achieved. Furthermore, according to the invention, it is only possible to pre-treat part of the surface of the non-conductive material as substrate with the chitosan-containing treatment liquid. In this case, the catalyst is selectively formed only on the pretreated part(s) of the substrate surface, thereby enabling efficient performance of partial electroless precipitation in the subsequent step. According to the present invention, moreover, polyester resins such as polyethylene terephthalate, construction plastics, various alloys, etc., which have hitherto been difficult to precipitate free of particles, can be precipitated without particles in a satisfactory manner.

Furthermore, although the chitosan-containing treatment liquid is applied directly to the surface of the non-conductive substrate material in the preceding procedure, in certain cases an undercoat may be applied to the substrate surface before applying the treatment liquid thereon, any undercoat having the best adhesion to the substrate can be used without considering its compatibility with chitosan as mentioned above, although the number of steps is increased. Examples of undercoats include acrylic lacquers, acrylic coatings, and urethane-treated acrylic coatings that have excellent adhesion to plastics such as acrylic resins, ABS, polystyrene, polycarbonates, polypropylene, and polyesters.

In the catalysis reaction step, the substrate with the coating film formed as a catalyst metal-fixing support containing chitosan on its surface is simply immersed in a hydrochloric, nitric or acetic acid-acidified solution of hydrochloride, nitrate or acetate of a noble metal such as Pd, Pt, Au or Ag, or the like in a short period of time for complete fixation of catalyst cores only during one step which is different from the above-mentioned conventional sensitizing-activation or catalyst-accelerator methods. The representative noble metal salt is palladium chloride which can be used in the form of a solution (palladium chloride: 0.2-1 g/l, hydrochloric acid: 5 ml/l) such as an activation solution used in the sensitization-activation method. The chemisorption of palladium on the catalyst metal-fixing support containing chitosan is believed to be due to coordination bonds as illustrated in Fig. 4 [Baba et al; Bull. Chem. Soc. Japan 66, 2915 (1993)]. The chemisorption can prevent the palladium from falling off the support in the subsequent electroless precipitation step. Furthermore, the free amino groups in chitosan can be reacted with an aldehyde such as formaldehyde, salicylaldehyde, glutaraldehyde, pyridine-2-aldehyde, thiophene-2-aldehyde or 3-(methylthio)propionaldehyde to form a Schiff base, which can then be reduced by sodium boron tetrahydride or the like to form a catalyst metal fixing support on which the catalyst metal can be formed in a more selective and stronger manner, although it depends on the type of catalyst metal (see Fig. 5).

Next, the substrate with said catalyst metal-fixing support containing chitosan and with the catalyst metal formed is then immersed in a particle-free precipitation bath of Cu, Ni, Co, Pd, Au or an alloy thereof, after which the electroless precipitation which has excellent adhesion to the substrate can be obtained continuously by the reducing effect of the fixed catalyst cores located only on the part or parts of the substrate surface where the catalyst metal fixing support is present. The non-conductive material can thus be metallized in accordance with the desired purpose.

According to the sensitizing-activation method, catalyst-accelerator method, etc. as catalyzing methods in the conventional electroless plating processes, a catalyst metal is formed on the surface of a substrate that is microscopically roughened by chemical etching through a series of reaction steps effected on the surface of the substrate and by means of physical adsorption of the reaction product on the substrate surface, with the result that the rest of the catalyst metal is so unstable that in many cases it causes adverse effects such as adhesion failure during the course of the precipitation formation. In contrast, according to the present catalysis process, a treatment liquid containing chitosan is applied in the form of a coating on the surface of a substrate to form a kind of catalyst metal-fixing support on which the catalyst metal is strongly supported by chemisorption thereof, which thus enables the formation by electroless precipitation of a uniform metal deposit with good adhesion. Furthermore, the catalyst is only carried on the pre-treated part or parts of the substrate surface on which the treatment liquid has been applied so that the partial electroless precipitation can be achieved. Furthermore, chemical etching can be avoided so that the number of steps can be reduced and so that the waste treatment can be simplified, and this greatly contributes to an improvement in terms of environmental problems. Fig 1 is a flow diagram of a conventional electroless precipitation process comprising catalysis according to the sensitizing-activation method; Fig 2 is a flow chart for a conventional particle-free precipitation process comprising catalysis according to the catalyst-accelerator method; Fig 3 is a flowchart for a particle-free precipitation process where catalysis is carried out through the formation of a catalyst metal-fixing carrier containing chitosan according to the present invention; Fig 4 is an illustration showing the adsorption mechanism for palladium under the influence of chitosan; and Fig 5 is an illustration showing the adsorption mechanism for palladium under the influence of a chitosan derivative.

Preferred designs

Example 1

Chitosan (SK-10 manufactured by San-Ei Chemical Industries, Ltd) was dissolved in a 1% solution of acetic acid to form a 1% w/v solution of chitosan, which was then diluted with methanol to obtain a pretreatment liquid containing 0 .5% chitosan. The pretreatment liquid was applied to Japanese paper (300 mm<2 >mulberry wood paper and produced by Dai-Inshu Seishi Kyogyo Kumiai) using a spray or a brush, and then dried under forced air movement at 50°C for 1 hour.

The coated paper was then immersed in a solution of palladium chloride (PdCl2-2H2O: 0.3 g/l, hydrochloric acid: 5 ml/l) for 30 minutes, then washed with water, and then subjected to electroless copper precipitation in a precipitation bath with a composition as shown in Table 1.

pH 12.5, liquid temperature: 60°C stirring with air.

As a result, uniform copper deposition could be achieved on the entire surface of the Japanese paper in the case of applying the pretreatment solution to the entire paper surface, and only on a part of the surface of the Japanese paper in the case of partially applying the pretreatment solution to the paper surface.

Example 2

Chitosan (SK-100, Lot. 414-05; manufactured by San-Ei Chemical Industries, Ltd) was dissolved in a 1% solution of acetic acid to form a 1 w/v% solution of chitosan which was then mixed with 1 vol /vol% of a solution of salicylaldehyde (manufactured by Kishida Chemical Co., Ltd) diluted with a 10-fold amount of methanol to form a Schiff base, and further diluted with methanol after 1 hour to obtain a treatment liquid containing 0 .5% chitosan.

On the other hand, an alumina ceramic substrate (99.9%) was subjected twice to ultrasonic cleaning with distilled water for 5 minutes, further subjected to ultrasonic cleaning with methanol, and then dried to obtain a cleaned test piece.

The test piece was immersed in the above treatment liquid and then dried at 120°C for 30 minutes. Next, the test piece was immersed in a 0.5% solution of dimethylaminoborane to reduce the Schiff base with it, then immersed in a solution of palladium chloride (PdCl2-2H20: 0.03 g/l, hydrochloric acid: 5 ml/l) for 2 minutes, then washed with water and dried. At this step, the palladium adsorption was measured according to the procedure below. 100 ml of a 1% solution of nitric acid was added to the test piece and heated to dissolve the palladium which was thereby removed from the test piece. The solution was further heated to cause evaporation, then placed in a 50 ml graduated measuring cylinder in which distilled water was placed up to the marked line on the cylinder. The resulting solution was placed in a graphite pyrolysis tube and combusted at a combustion temperature of 2600°C for 3 seconds using a spectroscopic atomic absorption analyzer (AA-670G, manufactured by Shimadzu Seisakusho Ltd) and a graphite furnace atomizer (Model GFA-4) to measure the absorbance of palladium from which the palladium adsorption was calcd. As a result, it was found that the amount of adsorbed palladium on the catalyst metal fixation support containing chitosan was 11.5 µg per test piece in this example.

Next, another test piece was immersed in the treatment liquid and then in the solution of dimethylaminoborane in the same manner as described above, further immersed in the solution of palladium chloride for 2 minutes, then washed, and dried and subjected to electroless nickel precipitation in a precipitation bath with a composition as shown in table 2 for 30 minutes to achieve a uniform nickel precipitation.

Example 3

Chitosan (SK-100, Lot. 802-05; manufactured by San-Ei Chemical Industries, Ltd) was dissolved in a 1% solution of acetic acid to form a 1% w/v solution of chitosan. On the other hand, 20 parts of titanium oxide and 80 parts of aluminosilicate were mixed with and dispersed in 100 parts of an epoxy curing type acrylic resin (manufactured by Toray Industries, Inc) to obtain a solution which was then diluted with a mixed solvent of methanol/isopropyl alcohol/ ethyl acetate/butyl cellosolve (80:12:3:5) to form a 20% w/v solution. This solution was mixed with the above solution of chitosan in a ratio of 10:1 to obtain a pretreatment liquid containing chitosan as an active ingredient.

A specimen of ABS resin (50 mm x 150 mm x 2.0 mm-t) was prepared as a substrate, dried with a cloth moistened with isopropyl alcohol for degreasing and cleaning, and the specimen was then spray-coated with a solution prepared by adding 0.5 parts of an epoxy curing agent (DENACOL EX-850 manufactured by Nagase Chemicals, Ltd) to 100 parts of the above pretreatment liquid and diluting the resulting mixture with a 5-fold amount of a mixed solvent of butyl acetate/ethyl acetate/n-butanol/ toluene/butylcellosolve (20:25:20:25:10) followed by drying at 60°C for 1 hour.

The coated ABS resin piece was immersed in a palladium chloride solution

(PdCl2*2H20: 0.25 g/l, hydrochloric acid: 5 ml/l) for 3 minutes, then washed with water, then precipitated in a particle-free copper precipitation bath with a composition as shown in Table 1 for 30 minutes, and then further plated in a particle-free nickel precipitation bath with a composition which

shown in Table 3 for 5 minutes to obtain a uniform copper/nickel deposit with a thickness in the range of 1.5-2.0 nm.

It was determined that the copper/nickel precipitate thus obtained had a good appearance and an excellent adhesion in various property tests as shown in Table 4.

As for the adhesion, a 10 mm x 10 mm area of the copper/nickel deposit was cross-cut into 100 small squares each with a length of 1 mm and a width of 1 mm, and a cellophane adhesive tape was attached to the cross-cut area of the deposit and then torn off to judge the adhesion based on (number of remaining squares of precipitation/total number of squares).

Claims (4)

1. Process for catalysis by electroless precipitation of metals on the surface of a non-conductive material, e.g. plastic, glass, etc., characterized by the formation of a coating film including chitosan or a chitosan derivative on said surface, and then treatment of the coating film with a solution of a salt of a catalyst metal to effect chemisorption thereof on said coating film. 2. Method according to claim 1, characterized in that said coating film comprising chitosan or said chitosan derivative further comprises a resin. 3. Method according to claim 1 or 2, characterized in that the coating film comprising chitosan or said chitosan derivative further comprises an inorganic pigment. 4. Method according to claim 1, 2 or 3, characterized in that said salt of the catalyst metal is a salt of a noble metal.